1. Field of the Invention
This invention relates to a method and apparatus for controlling frequency of a thermally stable microwave oscillator bank.
2. Description of Prior Art
U.S. Pat. No. 4,629,999 teaches a phase-locked loop capable of generating stable frequency signals. Voltage controlled oscillators are sequentially coupled to an output of a phase sensitive detector by a multiplexer. Oscillator outputs are sequentially coupled in synchronism with a multiplexer, to a programmable divider which is coupled to an input of the phase sensitive detector. Another input is coupled to a reference oscillator.
U.S. Pat. No. 4,694,260 discloses a microwave frequency discriminator for transforming a frequency modulated signal into a low frequency demodulated signal, wherein the discriminator is used with microwaves. An oscillating circuit has a frequency controlled by an externally applied dc voltage which is applied to a controlled circuit, such as a circuit including varactor diodes. The '260 patent discloses neither multiple microwave oscillators nor thermally coupling or frequency tracking abilities of such microwave oscillators.
U.S. Pat. No. 3,614,640 discloses a frequency discriminator which operates without inductive components. A signal is divided into two channels. One channel acts as an active filter which connects to a positive detector having an output which is transmitted to a mixing device. The other channel acts as an active filter which connects to a minus detector having an output which is also transmitted to the mixing device. Each active filter uses no inductors but has a transfer function equivalent to that of a tuned circuit.
U.S. Pat. No. 3,868,606 discloses a highly stable HF and VHF source that uses a Q-multiplied quartz crystal resonator which includes a frequency discriminator section of an AFC stabilization loop, to provide a low-noise spectrally pure frequency source.
U.S. Pat. No. 4,333,062 teaches a temperature-stabilized MIC solid-state oscillator which has two strip line resonators with chip capacitors positioned serially in the middle of the strip lines. The chip capacitors have linear capacitance temperature characteristics.
U.S. Pat. No. 4,345,221 teaches a signal generator with a digital temperature compensation circuit that has high and low frequency quartz oscillators. The low frequency oscillator is used for temperature compensation.
U.S. Pat. No. 3,787,612 discloses signal processing circuits for a telephone receiver which include acoustic surface wave devices for tuning and video demodulation. Surface wave discriminators have similar thermal characteristics and thermal exposure which cause temperature tracking of the frequency characteristics and thus temperature stabilizing of the receiver.
It is apparent from the teachings of the known prior art references that there is a need for a frequency controller that uses a plurality of electrically and thermally matched oscillators, as well as a reference oscillator and a thermally stable frequency discriminator, to produce modulated individual signals that can be combined into an intermediate frequency output signal.